Documentary asks us to try to determine where fiction ends and nonfiction begins. The best of all nonfiction makes you think about the relation between film and reality.

Donald O’Connor (left) and Gene Kelly in “Singin’ in the Rain.”(Associated Press/file)

A question that concerns or should concern every nonfiction or documentary filmmaker: What is documentary? What makes it different from scripted films or drama? It’s a seemingly simple question, but it really doesn’t have simple answers. The best that I can come up with is that in documentary, we can ask whether something is true or false. In drama, we cannot. After all, it’s all made up. But even that doesn’t seem to quite capture it. Is it stuff that is just simply recorded “as it is” — unrehearsed, unconstrained, unscripted?

No art form can give us truth on a silver platter. But it can present evidence in such a way that we can think about what is true and what is false. In this respect, there are elements of nonfiction in all of fiction filmmaking, and vice versa. There is, strictly speaking, no dividing line. No big sign welcoming you to Fiction Land like you might see crossing state lines on a highway. When you see a performance in a drama, what you are seeing, in effect, is a documentary record of a performance. When Donald O’Connor is dancing in “Singin’ in the Rain” (1952), you’re seeing a documentary record of Donald O’Connor dancing.

The films that I’ve picked for this article all speak to the relationship between nonfiction and fiction. It could be a sudden moment that seems unscripted and absolutely real, documentary-like. It could be a performance which is based not on any script, but on a casting choice, as, for example, with the neorealists. I see it in the films of Roberto Rossellini and Robert Bresson, who eschewed professional actors for everyday people they found on the street. But the neorealists and their techniques don’t have a purchase on truth. Klaus Kinski in Werner Herzog’s “Aguirre, the Wrath of God” (1972) is a perfect example of the coin’s reverse.

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After all, what is Aguirre but an experiment in what happens if you put Kinski in a conquistador suit and send him to the Amazon?

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Klaus Kinski in Werner Herzog’s “Aguirre, the Wrath of God.”

There are countless instances of movies where something real shines through all of the fiction. There’s such a moment in “Gun Crazy” (1950), the plot of which originated with a story by Mackinlay Kantor in the Saturday Evening Post. The protagonist, Bart, gets home from the Army and goes shooting with one of his childhood buddies (who has now become the sheriff). Bart is shooting with his own pistols, and is a more extraordinary marksman than ever before. After destroying all his targets, Bart cracks open a beer, takes a drink, and stammers, “Ugh . . . warm beer.”

It was never part of the script. The shooting took too long. The beer was never meant to be consumed. Oddly enough, the reality of that moment shines through the film. “Ugh . . . warm beer.”

Or take “Bright Leaves” (2003) by the documentary filmmaker Ross McElwee. McElwee endlessly speculates on the relationship between the fictional film “Bright Leaf” (1950) and the factual details of his own family history. As his interest in the film becomes more and more like an obsession, McElwee begins to wonder whether stars Gary Cooper and Patricia Neal were having their famed affair while “Bright Leaf” was filming, and how it might have affected their performances. Is there more to be read into Neal stroking Cooper’s waist during a passionate scene? Is this a stage kiss, or are we watching a real-life Hollywood tryst? Or is it both?

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Or “Call Northside 777” (1948). Is it a feature or a documentary? A feature film based on a real court case that tries to remain faithful to the original story. James Stewart stars. There’s a scene that mirrors what happened to me in making “The Thin Blue Line” (though I wasn’t aware of it at the time). Evidence of eyewitness identification is undermined by the fact that events could never have unfolded the way police said they did. The case collapses, and the evidence against the defendant is proved fraudulent. Very powerful, and also an example of how a piece of documentary evidence can undermine the basis of a story. Like an essay in epistemology. And the nature of photographic evidence.

Never say that documentary is only about style — whether it’s verité or something different. It’s about the search for the truth — thinking about what is true and what is false. As such, anything can be in service of that enterprise — any scrap of evidence, any piece of testimony. Reenactments, whatever. You are brought deeply into the relationship between a movie and the world.

My last example is Luis Buñuel’s “Land Without Bread” (1933). This short film juxtaposes documentary footage with ironic commentary that seems to undermine the images at the same time as it displays them. In that way it’s like countless movies where interviews and narrative don’t provide a coherent or consistent story. In an investigation, you don’t necessarily look for consistency but for a deeper understanding.

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Do I dare say that a good documentary is more interesting than a good feature film? I could, although I don’t think that is quite right either. A good feature film needs to be consistent. A good documentary shouldn’t be. What I’m hinting at is that in drama, as in life, attempts to uncover the truth behind a narrative always involve inconsistencies. It’s our job to sort these things out. Good documentary helps us to see how.

Errol Morris, whose films include “The Thin Blue Line” and the Oscar-winning “The Fog of War,” is based in Cambridge.